Digital measuring head

ABSTRACT

A digital measuring head in a measuring apparatus measures a work by making a contact element abut to the work. The digital measuring head includes an arm supported rotatably around a support point arranged on a base; a finger having the contact element at a tip end and mounted to a tip end portion of the arm; and a scale and a read head, one of which is provided at a rear end portion of the arm and the other of which is arranged on the base. Displacement of the contact element which contacts with the work is measured with the scale and the read head. Thereby, the measuring head can perform measurement in a wide range and becomes excellent in the actuation property and the temperature property, and masters for calibration of an indicated dimension are only used at an initial adjustment time.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a measuring head, and particularly to aseesaw type measuring head which is used in a machine control gauge, asurface roughness, and contour shape measuring apparatus, or the like.

2. Description of the Related Art

A linear voltage differential transducer (LVDT) is conventionally usedfor a sensor for measuring displacement of a contact element in a seesawtype measuring head used in a machine control gauge, a surface roughnessand contour shape measuring apparatus, or the like.

FIG. 22 is a schematic diagram of this conventional measuring head. Ameasuring head 110 shown in FIG. 22 is a measuring head for measuring anouter diameter, in which two seesaw mechanisms are combined. In theconventional measuring head 110, a moving amount of a contact element114 supported rotatably around a support point member 111 is measured asa change in voltage in an LVDT 115 provided in a side opposite from thesupport point member 111. The measurement value of the LVDT 115 isprocessed in a control unit 140, and obtained as the moving amount.

However, since the conventional measuring head 110 uses the LVDT for thesensor, the following problems exist. Namely, since the linear range ofthe LVDT is narrow, the measuring range is narrow (generally, about 1mm) and measurement in a wide range cannot be performed.

Moreover, two of a large master and a small master for calibration arenecessary for calibration of indicated dimensions. For example, when awork having a diameter of 30±0.01 mm is measured, a master having adiameter of 30.01 mm (large master) and a master having a diameter of29.99 mm (small master) are used to adjust sensitivity of the measuringhead.

Especially in the case of the measuring head having a size shiftfunction of making it possible to respond to a several kinds of workdiameters with one measuring head, the large and small masters arenecessary for each dimension to be measured. For example, in the case ofthe measuring head corresponding to the diameter difference of 20 mm,the zero point can be set at anywhere within the range of 20 mm, but themeasurement range is about 1 mm, and therefore when three kinds of worksare to be measured, the total of six large and small masters are needed.This is because the linearity of the LVDT differs according to thelocations.

Further, setting for measurement takes time due to change of arrangementand a calibration operation by the masters and the like. Since themeasuring head is unfavorable in the temperature property and adjustmentand inspection for correction thereof take much time, and it takes timeuntil the measuring head is stabilized after the power is turned on, itis unfavorable in actuation property. These are the problems.

In order to solve these problems, a digital measuring head adopting anoptical digital mechanism, which moves linearly, is proposed in JapaneseUtility Model Application No. 62-078462. However, with the contentdisclosed therein, a mechanism for converting a moving amount of acontact element supported rotatably around a support point member into amoving amount of linear movement needs to be provided.

However, there exists the problem that the mechanism for converting therotational movement into the linear movement has a complicatedstructure, which leads to raise the cost and deteriorates the responsefrequency due to increase in the mass of a moving portion and frictionof the link support point, and therefore it is not applicable to thesurface roughness measuring head which is required for high-speedresponse frequency property.

SUMMARY OF THE INVENTION

The present invention is made in view of these circumstances, and hasits objects to provide a digital measuring head which is capable ofperforming measurement in a wide range, excellent in actuation property,temperature property, and further, response frequency property, andneeds only a small number of masters for calibration of indicateddimensions, and to provide a digital measuring head which can be usedfor both a contour shape measuring apparatus and a surface roughnessmeasuring apparatus due to its capability of measurement in a wide rangeand excellence in high-speed response frequency property.

In order to attain the aforementioned object, the present invention isdirected to a digital measuring head in a measuring apparatus whichmeasures a work by making a contact element abut to the work, thedigital measuring head comprising: an arm supported rotatably around asupport point arranged on a base; a finger having the contact element ata tip end and mounted to a tip end portion of the arm; and a scale and aread head, one of which is provided at a rear end portion of the arm andthe other of which is arranged on the base, wherein displacement of thecontact element which contacts with the work is measured with the scaleand the read head.

According to the present invention, the displacement of the contactelement which contacts with the work is measured with the scale and readhead, and therefore the measuring head can perform measurement in a widerange, becomes excellent in the actuation property, temperature propertyand response frequency property, and in addition, masters forcalibration of indicated dimensions may be used only at the initialadjustment time.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature of this invention, as well as other objects and advantagesthereof, will be explained in the following with reference to theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures and wherein:

FIG. 1 is a sectional side view showing a digital measuring headaccording to an embodiment of the present invention;

FIG. 2 is a sectional side view showing a Σ type measuring head formeasurement of an outer diameter;

FIGS. 3( a), 3(b) and 3(c) are perspective views showing supportmembers;

FIG. 4 is a schematic diagram explaining measurement of a gearwheel;

FIG. 5 is a perspective view showing a refraction interference typescale unit;

FIG. 6 is a perspective view showing a moire scale unit;

FIGS. 7( a) and 7(b) are schematic diagrams explaining an arc patternscale;

FIG. 8 is a block diagram showing a moire scale unit;

FIG. 9 is a schematic diagram explaining read of moire fringes;

FIG. 10 is a plan view showing the moire fringes;

FIG. 11 is a schematic diagram explaining a null fringe type scale;

FIGS. 12( a), 12(b) and 12(c) are schematic diagrams explainingplacement of a scale;

FIG. 13 is a perspective view showing a modified example of theembodiment of the present invention;

FIG. 14 is a schematic view showing another modified example of thepresent invention;

FIGS. 15( a) and 15(b) are sectional views explaining principles of adamper;

FIGS. 16( a), 16(b) and 16(c) are sectional views explaining a damper ofan embodiment of the present invention;

FIG. 17 is a sectional view explaining a modified example of the damper;

FIGS. 18( a) and 18(b) are schematic views explaining a modified exampleof a piston;

FIG. 19 is a schematic diagram explaining an arc error;

FIG. 20 is a graph explaining an axis deviation error;

FIG. 21 is a graph explaining a correction value of the arc error; and

FIG. 22 is a schematic diagram showing a conventional measuring head.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of a digital measuring head according to thepresent invention will be explained in detail below in accordance withthe attached drawings.

FIG. 1 is a sectional side view showing a construction of the digitalmeasuring head according to the present invention. A digital measuringhead 10 comprises a base 21, a support point member 11, an arm 12, afinger 13, a contact element 14, a scale 15, a read head (not shown), apressurizing member 17, a damper 18, a boot 19, a case 22 and the like.

The support point member 11 is supported by a holder 20 and attached tothe base 21. The arm 12 is held by the support point member 11, and isrotatable in seesaw motion around the support point member 11. Thefinger 13 is secured at one end of the arm 12 by a screw, and thecontact element 14 to abut to a work W is attached to a tip end of thefinger 13. The scale 15 as a sensor is attached to the other end of thearm 12. The read head which reads graduations of the scale 15 is fixedto the base 21.

The arm 12 is provided with the pressurizing member 17 for pressing thecontact element 14 toward the work W. A compression spring can be usedfor the pressurizing member, but any member such as a tension spring andthe other elastic members may be used if only it can causes a rotationalforce to the arm 12. Further, the arm 12 is provided with the damper 18,which dampens jumping and vibration of the contact element 14.

The members other than the tip end portion of the arm 12, the finger 13,and the contact element 14 are covered with the case 22. The case 22 hasan opening, through which the tip end portion of the arm 12 protrudes.The opening is provided with the boot 19, which prevents dust and mistfrom entering an inside of the case 22.

Next, an operation of the digital measuring head 10 thus constructedwill be explained. First, the digital measuring head 10 is set for thework W. Since the contact element 14 is pressed to the work W by thepressurizing member 17, the contact element 14 is rotationally displacedaround the support point member 11 correspondingly to the size of thework W. When the contact element 14 is rotationally displaced, the scale15 attached to the arm 12 is also rotationally displaced. Thedisplacement amount of the scale 15 is measured with the read head,processed in an A/D converter unit 30, and sent to a control unit 40,whereby the size of the work W is obtained.

Since the arm 12 is provided with the damper 18, jumping and vibrationof the contact element 14 are prevented. Since the boot 19 is attachedto the opening of the case 22, a processing solution, mist, dust and thelike never enter the case 22.

The digital measuring head 10 shown in FIG. 1 is the measuring head formeasuring thickness and elevation change, a contour shape, and surfacecondition (hereinafter referred to as a Γ type measuring head). On theother hand, the digital measuring head 10 shown in FIG. 2 is a measuringhead which is made by combining two of the structures shown in FIG. 1 tobe symmetrical to measure an outer diameter of the work W. As for thismeasuring head, other than the outer diameter measuring head shown inFIG. 2, there is an inner diameter measuring head in which the contactelements 14 are attached in the reverse directions, and the pressurizingdirections of the pressurizing members 17 are reversed. The operationsof the outer diameter measuring head and the inner diameter measuringhead (hereinafter, referred to as the Σ type measuring head) arebasically the same as the Γ type measuring head shown in FIG. 1, andtherefore the explanation will be omitted.

Next, each component will be explained in more detail. A main bodystructure of the digital measuring head 10 is such that all of the maincomponents other than the finger 13 and the contact element 14 aremounted on the base 21 to be one unit and thereafter the unit is mountedto the case 22. The base 21 is made of the same material as the work Wor made of the material with the same thermal expansion coefficient asthe work W, whereby the measuring head excellent in temperature propertyis provided. This is because the work W expands or contracts if theambient temperature changes. If the linear expansion coefficients arethe same between the base 21 and the work W, the base 21 expands andcontracts with the fixed screw portion as the base point as the work W,and therefore the measurement error can be made small. Even with use ofthe same material, if each component is fixed to different locations ofthe case 22, the change amount differs in accordance with the locations,and therefore a large error occurs. The Σ type measuring head has thesymmetrical structure which is resistant to the influence of heat asshown in FIG. 2.

A cross spring support point as shown in FIG. 3( a) is used for thesupport point member 11. As the cross spring support, there is theconstruction in which two leaf springs are aligned in such a manner asto cross each other as shown in the illustration on the left-hand sideof FIG. 3( a), but in this embodiment, the integrated one by cutting bywire cut machining as shown in the illustration on the right-hand sideof FIG. 3( a) is used. This integrated cross spring support point hashigh rigidity and favorable repeatability though with high processingcost, and therefore this is suitable for the support point with highprecision.

As the support point member 11 other than those describe above, anL-shaped spring support point shown in FIG. 3( b), an elastic supportpoint shown in FIG. 3( c), a bearing or pivot support point and the likecan be used. The L-shaped spring support point is inferior to the crossspring in precision but is less expensive, and is the support pointcommonly used widely. However, it has the disadvantage that the supportpoint center moves following the rotating movement, and though it is ata level which does not matter when the LVDT is used for the sensor, itcauses an error which cannot be ignored when the sensor is a scale. Forthis reason, the relation between the rotating movement and the movingamount of the support point center is found by calculation, andcorrection is made by software.

The elastic support point has high precision but can take only a smallrotation angle, and therefore it can be used at low cost when themeasurement range is narrow. The bearing support point has no limitationon the rotation angle, but reduces in precision due to abrasion, andtherefore it needs to be replaced periodically.

The finger 13 is formed to be separate from the arm 12 and fixed to thearm 12 with the screw so that it can be replaced correspondingly to theshapes of works, but it may be integrated with the arm 12. Stainlesssteel is used as the material of the finger 13, but iron with rustprevention plating being applied, ceramics, and the like may be used. Asfor the sectional shape, the finger 13 has the circular section toreduce influence of pressure of coolant on measurement, because thecoolant splashes on the finger 13 when the work W which is worked whilethe coolant is applied to it is measured. However, the sectional shapeis not limited to a circular shape, and a square shape, an oval shapeand the like may be used.

As for the contact element 14, the screw type contact element 14 is usedso that fine adjustment in the height direction can be performed asshown in FIGS. 1 and 2. Another construction may be used in which thecontact element 14 is directly embedded in the finger 13 and adjustmentin the height direction is performed at a mounting portion of thefinger. In the case of Γ type measuring head, adjustment in the heightdirection can be performed by moving the entire measuring head.

Super steel or diamond is embedded in a tip end portion of the contactelement 14 which contacts the work W. Since the tip end portion needs tobe strong against abrasion, and super steel, diamond, ruby and the likeare generally used, and they are properly used in accordance with thematerial of the work W.

When a hard material such as iron and stainless steel is measured,inexpensive super steel is used, but when rotating work W is measured,diamond which is stronger against abrasion is used. When a soft materialsuch as aluminum, copper, and soft glass is measured, diamond whichslides smoothly is used, so that a damage and a dent are prevented fromoccurring by reducing a measuring force, and reducing the falling speedfrom a retracted state.

A round shape is usually used for the tip end shape of the contactelement 14. A radius of curvature of the round shape is not limited, butespecially when the work W which is under working in process ismeasured, the tip end shape with the small radius of curvature of about0.5 mm to 1.5 mm is used so as not to catch chips.

When the measured surface is rough, the value may be varied as a resultof catching fine roughness, and therefore the tip end shape with a largeradius of curvature for the purpose of equalization is used. In thiscase, the tip end having the radius of curvature of around 3 mm to 6 mmis normally used. In the case of the soft work W, the tip end having theradius of curvature of 3 mm or more is used for the purpose of reducingthe dent of the work W at the time of contact, and the tip end havingthe radius of curvature of about 3 mm to 6 mm is typically used.

When a discontinuous surface such as a gearwheel is measured, the tipend having the radius of curvature in the size to stride a groove isused as shown in FIG. 4, the radius of curvature in the paralleldirection with the groove is made about 1.5 mm, and the contact element14 in a boat shape as a whole is used.

When an outer diameter of a rotating gearwheel is measured, themeasurement value is varied due to the amounts of falling and jumpingbeing large if the small contact element 14 is used, and therefore thestructure, which can ride on peak from valley favorably, needs to beadopted. Even with the structure of striding the groove, falling andjumping cannot be constrained completely. Thus when the rotationincreases in speed, larger rattles are caused, or the tip end keptjumping up, and the tip end hardly contacts the gearwheel. Consequently,it is necessary to increase the effect of the damper 18 and control thecircumferential speed of the work W and the contacting andnon-contacting time. For measurement, the method for rotating the work Wone round or more and memorizing the maximum value is used.

Other than the above, the cylindrical super steel contact element 14 maybe used. In this case, a split is provided in a contact elementinsertion hole of the finger 13, and the contact element is fixed bysplit clamping. When the contacting portion with the work W is worn, itis rotated and fixed again so that the unused surface is used, wherebythe service life is extended. However, this circular contact element 14is difficult to be positioned in parallel with the work W, and thereforethis is used under the special condition such as in the case in whichthe space is so narrow that the round-shaped type contact element cannotbe used.

In the case of the contour shape measuring apparatus, the radius ofcurvature of the tip end of the contact element 14 is usually from 0.01mm to 1.0 mm, and in the case of the surface roughness measuringapparatus, it is usually from 0.002 mm to 0.010 mm.

FIG. 5 is a perspective view explaining a construction of the scalesection. The scale section is constructed by the scale 15 and the readhead 16 as shown in FIG. 5. The reflection type scale utilizingrefraction interference of light is used for the scale 15, which isfixed to the arm 12 rotating around the support point member 11. Theread head 16 is placed in the vicinity of the scale 15 to measure themoving amount of the scale 15. The read head 16 is constituted of alight emitting element as a light source (not shown), a collimate lensor a condensing lens, and a light receiving element as a light receivingdevice.

FIG. 6 is a perspective view showing a construction in the case of usinga transmission type moire scale is used for the scale 15. In the case ofthe transmission type moire scale, the scale 15 fixed to the arm 12 isthe main scale, and the read head 16 is constituted of an index scale16A placed close to the main scale, a light emitting element 16C as alight source, a collimate lens 16D, and a light receiving element 16B asa light receiving device.

In the aforementioned embodiment, the reflection type scale and thetransmission type moire scale are shown as the scale 15, but the presentinvention is not limited to them, and various kinds of scales such as atransmission type scale utilizing refraction interference, a reflectiontype moire scale or the like.

The scale pattern is formed to be a circular arc shape in either method,in either case of the reflection type scale or of the transmission typescale. Therefore, as shown in FIG. 7( a), the scale 15 is placed so thatthe center of the arc of the read point and the center of the supportpoint member, which is the center of rotation of the arm 12, correspondto each other. If this positioning is out of the alignment, an eccentricerror occurs, and therefore accurate adjustment is made.

Since the scale pattern is in an arc shape, the pattern pitch differs inaccordance with the read positions. FIG. 7( b) is an enlarged view ofthe arc pattern of the scale 15, and as shown in the same drawing, thepattern pitches differ as a, b, and c in accordance with the positionsin the radius direction. Consequently, it is designed that the desiredpitch is obtained at the read position.

As for an origin on reading the scale 15, a mechanical origin such as alimit switch may be provided, but the origin is provided on the scale asshown in FIG. 7( a) in this embodiment. As for the origin on the scale,the method for forming one slit on the scale and optically measuringwhether this slit passes or not is adopted, but instead of the slit, apattern may be formed. When a count error occurs due to noise or thelike, zero setting has to be redone using a master in the scale 15 inwhich the origin is not formed, but in the case with the origin beingformed, resetting is done by making the slit pass the origin once.

In the aforementioned embodiment, the read head 16 is fixed and thescale 15 is rotationally moves, but the present invention is not limitedto this, and the scale 15 may be fixed and the read head 16 may berotationally moved. In this case, some device is needed in wiring of asignal line from the read head 16.

In the moire scale method, light emitted from LED that is the lightemitting element 16C is made parallel light beams at the collimate lens16D to illuminate the index scale 16A and the scale 15 that is the mainscale. Moire fringes occurring by combination of the index scale 16A andthe scale 15 that is the main scale is read by a photodiode that is thelight receiving element 16B.

FIG. 9 is an explanatory view of the case of a linear type moire scale.The moire fringes occur by tilting the index scale 16A with respect tothe main scale. The pitches of the moire fringes depend on the pitch ofthe pattern and the tilt angle between both scales. Four photodiodes areused and the positional relationship between the moire fringes and thephotodiodes is adjusted so that four phase signals of 0°, 90°, 180° and270° can be obtained from the moire fringes. The number of thephotodiodes is not limited to four, and for example, 24 of photodiodesmay be used, and six groups of signals each obtaining four phase signalsmay be averaged.

In the case of the arc scale, the patterns of the main scale and theindex scale 16A are formed to be the same, and both of them are placedto tilt, whereby arc moire fringes occur in the perpendicular directionto the pattern as shown in FIG. 10. When the main scale and the indexscale 16A having the same arc patterns are displaced in the radiusdirection to be placed, the moire fringes occur in the paralleldirection with the pattern. When the patterns of the main and index areformed to be the different patterns in the pattern pitches or arcradiuses, and are displaced in the radius direction similarly, the moirefringes parallel with the patterns also occur.

Other than the aforementioned method, in the linear type, there is themethod in which a plurality of grids with different phases are formed onthe index scale 16A, a null fringe type in which the entire position ofthe phase changes to bright and dark according to the combination withthe main scale for each phase is constructed, and photo diode cells areplaced behind the respective grids displaced in phase. When the nullfringe type is applied to the arc scale, it is necessary to form thepatterns at a plurality of positions of the index scale 16A withdifferent arc radiuses by displacing them in phase, and therefore thepatterns are formed by changing the pitches as well as the phases inaccordance with the positions with different arc radiuses as shown inFIG. 11.

FIGS. 12( a), 12(b) and 12(c) show examples of three kinds of mountingpositions of the scale 15. FIG. 12( a) shows the case in which the scale15 is provided on the same straight line as the finger 13, FIG. 12( b)shows the case in which it is provided in a direction at 45° withrespect to the finger 13, and FIG. 12( c) shows the case in which it isprovided in the orthogonal direction to the finger 13. The mountingposition of the scale 15 may be at any angle if only the center ofrotation of the support point member 11 corresponds to the center of thearc of the arc pattern of the scale 15, and the arm 12 and the finger 13may be in any shape in their middle portions. However, when a force fromthe arrow direction as in FIG. 12( a) is applied to the contact element14, it hardly causes an error in FIG. 12( a), but it directly leads to areading error in FIG. 12( c).

FIG. 13 shows the case in which one scale unit is shared in the Σ typemeasuring head. In this embodiment, the scale 15 is mounted to the arm12 for the upper contact element 14, and the read head 16 is mounted tothe arm 12 for the lower contact element 14, as shown in FIG. 14. Thesupport members 11 and 11 of both the arms 12 and 12 are placed so thatthe centers of rotation are positioned on the same straight line. Theconstruction in this case has not only the merit that construction ismade at low cost with use of only one set of scale unit, but also themerit that reading resolution is doubled.

The above-described construction includes a modified example applied tothe inner diameter measuring head in which the directions of the contactelements 14 and 14 are reversed and the directions in which themeasuring pressure is applied are reversed.

As a modified example of the placement structure of the scale unit,there are the method in which a grid is formed on a member having around-shaped surface provided at a rear end of the arm 12, and themethod in which a tape scale is adhered to the round-shaped surface andan interferometer is placed at a position opposing to the round-shapedsurface, as shown in FIG. 14. In the case of the modified examples, thethickness of the entire measurement head can be made smaller, but thesize in the longitudinal direction increases.

FIGS. 15( a) and 15(b) are sectional views showing principles of thedamper 18. The damper 18 is for damping the jump of the contact element14 when the work is rotated, and is especially necessary when adiscontinuous surface such as a gearwheel is measured. The damper 18 isa dash pot type damper, and a piston 18B attached to a shaft 18Cperforms piston movement inside a cylinder 18A filled with a dampermaterial 18E. Pressure difference occurs at both sides of the piston 18Bbased on resistance of a channel when the damper material 18E passesthrough the narrow channel following the movement of the cylinder 18A,and the pressure difference works on the piston surface and givesresistance to the piston movement. FIG. 15( a) shows the case in which ahole 18D formed in the piston 18B becomes the channel, and FIG. 15( b)shows a case in which clearances between the piston 18B and the cylinder18A become the channels.

FIGS. 16( a), 16(b) and 16(c) are sectional side views explaining thedamper 18 in this embodiment. The damper 18 is constituted of thecylinder 18A, the piston 18B, the shaft 18C, the damper material 18E, aboot 18F, a hinge pin 18G and the like, as shown in FIGS. 16( a), 16(b)and 16(c). The cylinder 18A is in a cylindrical shape, and silicone oilis filled inside the cylinder 18A as the damper material 18E. The piston18B has a spherical body with a slightly smaller diameter than an innerdiameter of the cylinder 18A, is connected to one end of the shaft 18Cand reciprocates inside the cylinder 18A.

The other end of the shaft 18C is rotatably connected to the arm 12 viathe hinge pin 18G. The boot 18F is attached to an upper portion of thecylinder 18A, so that the damper material 18E filled inside does notleak out. The damper material 18E is not limited to silicone oil, butmay be water, engine oil, or the like, or may be air. In the case inwhich the damping effect is intended to increase, the material with highviscosity is used.

The shaft 18C is rotatably connected to the arm 12 via the hinge pin18G, but since the piston 18B has a spherical body, the damping effectcan be obtained even if the hinge pin 18G is omitted and the shaft 18Cis fixed to the arm 12 in the case of the measuring head with fewermeasuring strokes.

FIG. 16( a) shows a state in which the arm 12 is raised at its rightside, FIG. 16( b) shows the arm 12 in a horizontal state, and FIG. 16(c) shows a state in which the arm 12 is lowered at its right side. Asshown in the drawings, a change in volume following coming and going ofthe shaft 18C is absorbed by expansion and contraction of the boot 18F.

FIG. 17 shows a modified example of the damper 18. In this modifiedexample, the shafts 18C and 18C with the same diameters are provided atboth sides of the spherical piston 18B, and the boots 18F are attachedat both sides. According to this modified example, by the amount bywhich the shaft 18C is pushed in, the shaft 18C at the opposite sidesticks out, or by the amount by which the shaft 18C is pulled out, theshaft 18C at the opposite side goes in, and therefore there is no volumechange and no need that the boots 18F expands and contract, thusincreasing durability of the boot 18F.

In order to realize a subtle damping effect, it is necessary to considera cavitation phenomenon. This is the phenomenon in which bubbles andvoids occur when the portions at low pressure are formed in a flow, andthe phenomenon occurs when the viscosity of fluid, the clearance and thespeed satisfy a certain condition. This phenomenon is the most likely tooccur in the case with high viscosity and narrow clearance at highspeed, and it becomes a hindrance when the damping effect is desired tobe high and correspond to the abrupt displacement of the contact element14.

FIGS. 18( a) and 18(b) show a state in which a plurality of grooves areradially formed on a surface of the spherical piston 18B, and is themodified example in which the aforementioned cavitation phenomenon isprevented from occurring. FIG. 18( a) is a side view, and FIG. 18( b) isa plan view. In this case, the diameter of the spherical piston 18B ismade slightly small to widen the clearance, and increase viscosityresistance by the grooves by that amount. However, the resistance forceby the viscosity resistance is much smaller as compared with theresistance force by the clearance, and therefore the viscosity of thedamper material 18E is made high.

In the above-described embodiment, the piston 18B is made spherical, butthe shape in which the top and bottom of the sphere are sliced and onlythe side surface is made a spherical surface may be adopted, or a barrelshape may be adopted, and various kinds of shapes may be used if onlythey are the shapes capable of corresponding to the tilt of the shaft18C.

The correction of an arc error which matters in the seesaw typemeasuring head will be explained. By using the scale 15 for the sensor,wide-range measurement is made possible, but for example, in the outerdiameter measurement, following the arc movement of the contact element14, a contact point of the contact element 14 with the work W isdeviated from an axis of the work W by E as shown in FIG. 19. If thedeviation is changed linearly from the minimum diameter of the work W tothe maximum diameter, calibration may be made with two masters with theminimum diameter and the maximum diameter, but it is actually changed ina cubic equation curve form, the correction is made with software. Thelocus of the contact point of the contact element 14 with the work W iscalculated from the positional relationship of the support point of thearm 12 and the work W, and an error of each measured diameter with ascale zero point as a reference is calculated and corrected.

FIG. 20 shows a graph of an axis deviation error during outer diametermeasurement when a radius R1 of the work W is 4 mm and a tip end radiusR2 of the contact element 14 is 1.5 mm. The correction value which iscalculated based on the above when the distance from a center of thesupport point member 11 to the tip end of the contact element 14 is 155mm is shown in the graph in FIG. 21.

In the Σ type measuring head, deviation of the abutment points of theupper and lower contact elements 14 and 14 are eliminated, and the axisheight of the work W and the center position in the vertical directionof the Σ type measuring head are aligned, whereby only one master usedin the calibration is needed, and is used only at the time of adjustmentin the factory, and the master is not needed at a customer site.

The digital measuring head 10 of the present invention is provided witha retract device (not shown). The retract device is a release mechanismwhich brings the finger 13 into an open or closed state when the digitalmeasuring head 10 is moved to the measuring point. In this retractdevice, a mechanism for pushing and pulling a lever with use of an aircylinder or a latching solenoid is used.

A mechanism, which uses a motor other than the air cylinder and thelatching solenoid, and pushes and pulls the lever, interlocked with themovement of putting the digital measuring head 10 out and in, may beadditionally provided. When the control of the falling speed of thecontact element 14 from the retract state is needed, a restrictor isattached to the air cylinder and the falling speed is controlled by aflow of air. The retract device may be an internal type which is loadedin an inner portion of the digital measuring head 10, or may be anexternal type provided at an outer portion.

If a sudden temperature change is given to the digital measuring head10, condensation sometimes occurs. In this case, troubles such asinsufficient electrical insulation and formation of rust in the wiringportions, poor optical reading of the scale 15, and reduction in thesignal level are caused. Consequently, in this embodiment, air purgemethod is adopted. This is the method for preventing condensation byblowing dry air into the case 22 of the digital measuring head 10. Inthis case, air used in the retract device may be divided and used. Otherthan the air purge method, a double case structure method, a heaterincorporating method, and the like may be used.

As measurement items by the digital measuring head 10 of the presentinvention, there are an outer diameter, an inner diameter, an elevationdifference (height), size measurement such as thickness, surfacemeasurement such as roughness and swell, shape measurement, positionmeasurement, and geometrical tolerance such as coaxiality,concentricity, straightness, cylindricity, perpendicularity,parallelism, taper degree, contour degree of a line, contour degree of asurface, gradient, position degree, symmetry degree, radial run-out, andtotal run-out.

As explained thus far, according to the digital measuring head of thepresent invention, the scale is used for the sensor, and therefore themeasurement range of several ten millimeters is practically obtained ascompared with the measurement range of about 1 mm when using the LVDTfor the sensor, thus making wide-range measurement possible. It is notnecessary to prepare a master for calibration for each diameter of sizeshift, and the number of masters can be reduced, thus making it possibleto reduce set-up time and arranging time, and adaptation to productionof various kinds in various amounts is facilitated. Reduction in thenumber of masters gives a large merit in the aspects of production,management, delivery time, price and the like to both the makers and theusers.

Since the LVDT has an analogue signal, a zero point deviation and adrift occur, and waiting time for stability is needed, but in thedigital measuring head of the present invention uses the scale, andtherefore digital conversion is carried out immediately after readingthe scale, thus only requiring the electric circuit of digitalprocessing system, which carries out signal processing, making actuationmeasurement (cold start) possible, and improving actuation property.

Further, temperature property is remarkably improved as compared withthe LVDT. Since the LVDT has a large instrumental error, correction hasto be made by confirming the correction amounts one by one when thecorrection is made, which takes much time, but the scale has fewinstrumental errors, and therefore the number of man hours foradjustment and inspection can be substantially reduced.

In the case of the digital measuring head adopting the optical digitalmethod, which performs linear movement, a complicated mechanism forconverting the rotationally moving amount of the contact element intothe moving amount of linear movement, and response frequency isdeteriorated due to an increase in the mass of the movable parts,friction of the link support points, and the like, but the digitalmeasuring head of the present invention can perform measurement in awide range, and is excellent in high-speed response frequency property,thus making it possible to use the digital measuring head commonly inthe contour shape measuring apparatus, the surface roughness measuringapparatus and the like.

It should be understood, however, that there is no intention to limitthe invention to the specific forms disclosed, but on the contrary, theinvention is to cover all modifications, alternate constructions andequivalents falling within the spirit and scope of the invention asexpressed in the appended claims.

1. A digital measuring head in a measuring apparatus which measures awork by making a contact element abut to the work, the digital measuringhead comprising: an arm supported rotatably around a support pointarranged on a base; a finger having the contact element at a tip end andmounted to a tip end portion of the arm; and a scale and a read head,one of which is provided at a rear end portion of the arm and the otherof which is arranged on the base, wherein displacement of the contactelement which contacts with the work is measured with the scale and theread head based on at least one of refraction and reflection of light bythe scale.
 2. The digital measuring head of claim 1, wherein thereadhead further includes, an emitter adapted to emit light, and areceiver adapted to receive at least one of reflected and refractedlight.
 3. The digital measuring head of claim 1, wherein the scalereflects or refracts light emitted by the readhead.
 4. The digitalmeasuring head of claim 1, further comprising a pressurizing member, anda damper, wherein the pressurizing member is adapted to keep the contactelement in continuous contact with the work and the damper is adapted toreduce unwanted oscillations of the arm.
 5. A digital measuringapparatus comprising: a control unit; an analog to digital converter; anarm rotatably supported around a support point arranged on a base; afinger, including a contact element mounted to an end portion of thearm; and a scale and a read head, at least one of which is provided at arear end portion of the arm and the other of which is arranged on thebase; wherein displacement of the contact element which contacts with awork is measured with the scale and the read head based on at least oneof refraction and reflection of light by the scale.
 6. The digitalmeasuring apparatus of claim 5, wherein the readhead further includes,an emitter adapted to emit light, and a receiver adapted to receive atleast one of reflected and refracted light.
 7. The digital measuringhead of claim 5, wherein the scale reflects or refracts light emitted bythe readhead.
 8. The digital measuring head of claim 7, wherein thedamper is a dash pot type damper.
 9. The digital measuring head of claim5, further comprising a pressurizing member, and a damper, wherein thepressurizing member is adapted to keep the contact element in continuouscontact with the work and the damper is adapted to damp the jump of thecontact element.